There has been known such a specific polyol that the average number of moles of ethylene oxide added per active hydrogen and the primary hydroxidation ratio of terminal hydroxyl groups satisfy a specific relation. It is known that in producing a urethane foam, a soft polyurethane foam with superior vibration characteristics can be produced by making a polyol component contain this specific polyol (Patent Document 1).
On the other hand, cost reduction has been strongly demanded in recent years, and a lower density soft polyurethane foam has been desired for weight reduction. For example, in application to vehicles, reduction in the density of a soft polyurethane foam has also been required for weight reduction in order to cope with fuel efficiency requirements.
As a countermeasure to such demands for density reduction, the amount of water used as a foaming agent is liable to further increase. Increase in the amount of water used (Non-Patent Document 1, etc.) leads to increase in the amount of carbon dioxide gas generated in foam production and is thus effective for density reduction of a soft polyurethane foam. However, if the density of a foam decreases, foam hardness also decreases. Specific techniques for increasing the hardness of a soft polyurethane foam include, for example, a method involving increasing the amount of a crosslinking agent used (Non-Patent Document 1). However, such a method still has problems such as insufficient mechanical properties, such as elongation and tensile strength, of a soft polyurethane foam, and therefore a soft polyurethane foam with improved hardness and maintained mechanical properties has been demanded.